Sensor-less Brushless DC motors and methods for driving such motors, are known in the art. The reader not familiar with BLDC motors may refer to “A COMPARISON STUDY OF THE COMMUTATION METHODS FOR THE THREE-PHASE PERMANENT MAGNET BRUSHLESS DC MOTOR” by Shiyoung Lee, Ph.D., Pennsylvania State University Berks Campus, for a basic introduction of three widely used different commutation methods: trapezoidal (sixstep), sinusoidal and Field Oriented Control (FOC). Table 2 and FIG. 12 of this document show the major advantages & disadvantages of these algorithms. In short, Trapezoidal: low complexity, significant torque ripple; Sinusoidal: medium complexity, Low torque ripple; FOC: high complexity, Low torque ripple. The present invention focuses on sinusoidal motors.
In three-phase trapezoidal motor control, typically two phase windings are energized, and one phase winding is left open. As is very well known in the art, the open winding is typically used to measure the back-electromotive force (BEMF) voltage generated in the windings, when the motor is running. This voltage can be used to determine suitable moments for commutation. This principle is not applicable to sinusoidal control, because no phase winding is left open. Several techniques have been proposed in the art to determine suitable commutation points for sinusoidal motor control.
For example, in one of the prior art techniques, one of the windings is temporarily opened for a short while, sufficiently long for the BEMF to be measured, and is then closed again, for further applying a sinusoidal driving signal. The moment of opening the winding is preferably chosen close to a zero-crossing of the current, in order to keep the effects of the disturbance as small as possible. A problem with techniques based on BEMF is that they are not very reliable for starting the motor, because at startup the motor speed is very small.
There is always room for alternatives and improvements.